Unleaded 91 vs higher octane petrols

I have received very varied opinions as to what octane rating petrol should be used in my late model Nissan X-Trail with the 2.5- litre engine and CVT transmission. Some say there's no difference in use with between unleaded 91 and higher octane petrols, or you must use the higher rated octane petrols only, or use higher octane petrols for city driving and 91 octane for country driving. With the increasing cost of petrol what is regularly used significantly impacts on motoring budget and octane level of fuel regularly used on engine life and servicing also a major question too. Do you think the higher octane in the city and 91 octane for country driving would be ok? Or should I stay with higher octane petrols all round?